The high level of histone deacetylases (HDACs) in cancer cells change their epigenetics and protects them against genotoxic stress. We have studied a unique inhibitor of histone deacetylase (HDACI) valproate-valpramide of acyclovir (AN446) that upon metabolic degradation releases the HDACI- valproic acid. AN446 possesses anticancer activity, augments doxorubicin (Dox) killing of cancer cells, while protecting non-cancerous cells from Dox-toxicity. In the glioblastoma U251 xenograft model, AN446 inhibited HDAC activity in tumors, hearts and brains. It induced the greatest total H3 acetylation in tumors followed by the brains the hearts. This differential changes in histone modifications observed provides the basis for the drug selectivity. However, the unfolding picture was more complex and entailed modifications of specific lysine (K) residues. We examined changes in modifications of specific lysine residues on H3 including acetylation of H3K9, H3K18 and H3K56 and methylation of H3K9 and H3K27 after treatment with AN446. Acetylated H3K9 (AcH3K9) was highest in tumors and brains but was low in hearts. AcH3K18 was high in the tumor and brain and lower in the heart, in tumors it increased significantly by AN446, while in the heart or brain, the same treatment induced a significantly lower increase. The level of AcH3K56 was 2-fold higher in the tumors compared to brains and > 10-fold lower in the heart. AcH3K56 in the tumors was reduced significantly by AN446 but was unchanged in brains and slightly increased in the hearts. Methylation of H3K9 and H3K27 that were high in the tumors were decreased by AN446. The modifications of histones by AN446 and the interpretation of this complex language ("histone code") to tissue-specific changes will be discussed.
